1. Field of the Invention
The present invention relates to a tubular calibration unit for calibrating the outer or inner diameter of extruded plastic string bodies such as pipes.
2. Description of Related Art
In the production of extruded plastic pipes it is customary that the hot plastic compound is forced through an extruder nozzle, the diameter of which is somewhat larger than the desired pipe diameter, the extrusion taking place to a calibration tube unit, at the inlet of which the plastic pipe is narrowed to the diameter of this unit, this diameter corresponding fully or almost to the desired pipe diameter. The metallic calibration tube is cooled by cold water such that during its throughflow the plastic compound solidifies sufficiently to be stabilized with a corresponding pipe diameter as it leaves the unit. Thereafter the pipe is passed through a cooling zone, normally of a length larger than that of the calibration tube, such that at the outlet from this zone the pipe is stabilized sufficiently for co-operation with a pulling station effecting the pull out of the pipe from its initial formation in the calibration tube. Thereafter the pipe can be passed further to be reeled or cut into desired lengths.
Normally, the production tolerances should be rather narrow, both for satisfying certain minimum requirements and for achieving this without overdosage of plastic.
In practice it happens that for several reasons changes in the process conditions may occur, such that for the desired optimizing of the produced pipe it may be required to use a slightly changed calibration diameter. In such cases it is necessary to stop the process and replace the calibration tube by another such tube with a diameter deviation of, maybe, only one or half a millimeter.
Such a change, therefore, implies a production stop and often scrapping of many meters of produced pipe, so it would be highly desirable if it was possible to use calibration tubes with adjustable diameter, such that required adjustments could be effected currently, during operation.
The problem has earlier been considered in connection with narrow calibration rings, confer GB-1,349,436, where it has been possible to use a split ring with such a pronounced screw shaped split that the ring, seen in the direction of throughflow, will be unbroken annular, whether it is squeezed to a more or less advaced closing of the oblique split. Also, from U.S. Pat. No. 4,578,025 is known a construction comprising a row of mutually widely separated calibration rings provided at the outer end portions of funnel shaped elements and being controllable to undergo a more or less pronounced, resilient expansion or contraction.
These solutions, however, are unusable when it is a condition that the calibration unit be a heat/cold transferring, regular tube, preferably having a length up to several times its diameter. This is confirmed by practice, where it has been necessary to effect the said problematic total change out of the calibration tubes.